1. Field of the Invention
The present invention relates generally to a technique of controlling the charging of a secondary battery, particularly to a technique of preventing a secondary battery, which is used for applications in which the secondary battery is charged and discharged repeatedly in a charged but not fully charged state, from being overcharged by controlling the charging of the secondary battery.
2. Related Background Art
Examples of the secondary battery include lead batteries, nickel-cadmium (Nixe2x80x94Cd) batteries, nickel metal-hydride (Nixe2x80x94MH) batteries, and lithium ion batteries. Such batteries have a characteristic allowing them to be connected to an external power source to be charged with a predetermined current supplied from the power source after consumption of their electric power. These batteries have been used for various equipment, making good use of such a characteristic. For instance, such a battery may be mounted on a vehicle to supply electric power to an ignition plug of its engine. Recently, such a battery also is used as a main power source for driving an electric motor in a so-called hybrid electric vehicle (HEV) provided with an engine and the electric motor.
Every battery has its own limit in terms of the chargeable amount of electric power. Therefore, it is necessary to control the charging amount so that the battery is charged in a range not exceeding the chargeable capacity. When the battery is charged with an amount of electric power exceeding the limit (overcharged), there has been a possibility that the lifetime of the battery may be shortened. In addition, the detection of overcharge is indispensable for securing driving ability while a vehicle is driven.
Conventionally, a general method as a method of detecting overcharge of a battery includes detecting temperature variations in a battery during charging and determining that the battery has reached a fully charged state at a time a temperature variation per unit time, i.e. a temperature gradient, increases abruptly.
In the case of a battery mounted on, for example, a hybrid electric vehicle, however, the battery may be charged and discharged repeatedly while the vehicle is driven. In the hybrid electric vehicle, when the output of its engine is higher than the required power for driving, a generator is driven with the surplus power to charge the battery. On the contrary, when the output of the engine is lower than the required power, an electric motor is driven with the electric power of the battery to output the power covering a shortage of the required power. In this case, the battery is discharged. Such repeated charge and discharge are carried out according to the driving state of the vehicle, the charged state of the battery, and an operation of a driver.
Generally, a battery generates the Joule heat due to the loss caused by internal resistance (DC-IR) of the battery when the battery is charged and discharged. The amount of the heat thus generated is proportional to the square of an effective value of a current. Therefore, when charge and discharge are repeated as in the case where the battery is mounted on a hybrid electric vehicle, the temperature gradient abruptly changes even when the battery has not reached a fully charged state. Therefore, a wrong determination that the battery has reached the fully charged state may be made to stop the charging. As a result, the battery cannot be charged sufficiently and there is a possibility of troubles such that the battery goes flat.
As a method of solving the above-mentioned problem, for example, JP 11(1999)-299124 discloses a method including detecting a temperature gradient during charging caused in connection with the charging from the temperature gradient indicating a temperature increase per unit time or a temperature gradient during charging, calculated by subtracting a corrected amount of the temperature gradient caused by discharge from the temperature gradient indicating a temperature increase per unit time, and terminating the charging of a secondary battery when the temperature gradient is larger than a predetermined temperature gradient preset according to a charged state of the secondary battery.
However, in a working environment of a hybrid electric vehicle or an electric vehicle, the current values in charging and discharging the secondary battery are large and therefore the amount of heat generated by the internal resistance (DC-IR) of the secondary battery is large. In addition, the charging current and discharged current change instantaneously and heat is generated with a time difference. Therefore, the cause of the heat generation cannot be specified from instantaneous values of the currents. For such reasons, it has been difficult to detect overcharge by separating the heat generated by the DC-IR to determine the temperature gradient caused by a chemical heat-generating reaction alone upon overcharging.
With the foregoing in mind, it is an object of the present invention to provide a charging control device and method that reliably can detect overcharge by separating heat generated by internal resistance and heat chemically generated upon overcharging in a secondary battery.
In order to achieve the above-mentioned object, a charging control device according to the present invention is a device for controlling the charging of a secondary battery used for applications in which the secondary battery is charged and discharged repeatedly in a charged but not fully charged state. The charging control device includes a battery temperature detecting section, a temperature gradient operation section, and a temperature gradient determination section. The battery temperature detecting section detects the temperature of the secondary battery every first predetermined time. The temperature gradient operation section calculates a temperature gradient indicating a temperature increase per unit time for every second predetermined time that is longer than the first predetermined time, based on the temperature detected by the battery temperature detecting section. The temperature gradient determination section determines whether the temperature gradient calculated by the temperature gradient operation section is larger than a predetermined temperature gradient threshold preset according to a charged state of the secondary battery. When the temperature gradient determination section determines that the temperature gradient calculated by the temperature gradient operation section is larger than the predetermined temperature gradient threshold N times in a row (where N is a natural number), a fully charged state of the secondary battery is detected.
According to this charging control device, based on the temperature detected every first predetermined time, the temperature gradient operation section calculates the temperature gradient every second predetermined time that is longer than the first predetermined time, and then a fully charged state of the secondary battery is detected when the temperature gradient determination section determines that the temperature gradient calculated exceeds the predetermined temperature gradient threshold N times in a row (where N is a natural number). Thus, the influence of the heat instantaneously generated due to the repeated charge and discharge of the secondary battery while a hybrid electric vehicle or the like is driven is removed and the heat generated by the internal resistance and the heat chemically generated upon overcharging in the secondary battery are separated. Consequently, the overcharge can be detected reliably.
This can prevent the deterioration and life-shortening of the secondary battery due to the overcharge. In addition, the secondary battery can be used up to a range in which a high level of charging is required, and therefore the driving ability can be secured easily while a vehicle is driven.
In the charging control device, it is preferable that the temperature gradient operation section takes an operation result as being valid when an average load current of the secondary battery indicates a direction of charging the secondary battery.
According to this configuration, the temperature gradient calculated is used for overcharge detection when the average load current of the secondary battery indicates the charging direction, so that overcharge further can be detected reliably.
In the charging control device, it is preferable that a temperature gradient threshold preset according to at least one of the magnitude of an average load current of the secondary battery and cooling power of the secondary battery is used as the predetermined temperature gradient threshold.
According to this configuration, the temperature gradient threshold is set according to at least one of the magnitude of the average load current and the cooling power of the secondary battery, so that overcharge further can be detected accurately.
Preferably, a value of the average load current is an average value of load currents detected every first predetermined time during every second predetermined time.
In the charging control device, it also is preferable that the second predetermined time is longer than a time, detected every first predetermined time, of variations in load current due to charge and discharge of the secondary battery.
According to this configuration, the influence of the heat instantaneously generated due to the repeated charge and discharge of the secondary battery while a hybrid electric vehicle or the like is driven is removed, and the heat generated by the internal resistance and the heat chemically generated upon overcharging in the secondary battery are separated. Consequently, the overcharge can be detected reliably.
Furthermore, in the charging control device, preferably, the temperature gradient operation section determines the temperature gradient by a moving average operation.
According to this configuration, the temperature gradient is determined not with a simple average but with a moving average, so that overcharge can be detected further accurately.
In order to achieve the above-mentioned object, a charging control method of the present invention is a method for controlling the charging of a secondary battery used for applications in which the secondary battery is charged and discharged repeatedly in a charged but not fully charged state. The method includes detecting the temperature of the secondary battery every first predetermined time, calculating a temperature gradient indicating a temperature increase per unit time for every second predetermined time that is longer than the first predetermined time, based on the temperature detected, determining whether the temperature gradient thus calculated is larger than a predetermined temperature gradient threshold preset according to a charged state of the secondary battery, and detecting a fully charged state of the secondary battery when it is determined that the temperature gradient calculated is larger than the predetermined temperature gradient threshold N times in a row (where N is a natural number).
According to this charging control method, based on the temperature detected every first predetermined time, the temperature gradient is calculated every second predetermined time that is longer than the first predetermined time. When the temperature gradient thus calculated exceeds the predetermined temperature gradient threshold N times in a row (where N is a natural number), a fully charged state of the secondary battery is detected. Thus, the influence of the heat instantaneously generated due to the repeated charge and discharge of the secondary battery while a hybrid electric vehicle or the like is driven is removed and the heat generated by the internal resistance and the heat chemically generated upon overcharging in the secondary battery are separated. Consequently, the overcharge can be detected reliably.
This can prevent the deterioration and life-shortening of the secondary battery due to overcharge. In addition, the secondary battery can be used in a range up to a range in which a high level of charging is required, and therefore the driving ability can be secured easily while a vehicle is driven.
In the charging control method, it is preferable that an operation result of the temperature gradient is taken as being valid when an average load current of the secondary battery indicates a direction of charging the secondary battery.
According to this method, the temperature gradient calculated is used for the overcharge detection when the average load current of the secondary battery indicates the charging direction, so that overcharge further can be detected reliably.
In the charging control method, it also is preferable that a temperature gradient threshold preset according to at least one of the magnitude of an average load current of the secondary battery and cooling power of the secondary battery is used as the predetermined temperature gradient threshold.
According to this method, the temperature gradient threshold is set according to at least one of the magnitude of the average load current and the cooling power of the secondary battery, so that overcharge further can be detected accurately.
Preferably, a value of the average load current is an average value of load currents detected every first predetermined time during every second predetermined time.
In the charging control method, it is preferable that the second predetermined time is longer than a time, detected every first predetermined time, of variations in load current due to charge and discharge of the secondary battery.
According to this method, the influence of the heat instantaneously generated due to the repeated charge and discharge of the secondary battery while a hybrid electric vehicle or the like is driven is removed and the heat generated by the internal resistance and the heat chemically generated upon overcharging in the secondary battery are separated. Consequently, the overcharge can be detected reliably.
Moreover, in the charging control method, it is preferable that the temperature gradient is determined by a moving average operation.
According to this method, the temperature gradient is determined not with a simple average but with a moving average, so that overcharge further can be detected accurately.